Conventional container coatings may be derived from a thermally curable formulation that includes particles of a thermoplastic material (typically vinyl chloride polymers such as, for example, poly(vinyl chloride) (“PVC”)) in an organic solvent. When these coatings are applied to a substrate and cured, the thermoplastic material can degrade and discolor. Degradation products such as, for example, hydrochloric acid (“HCl”) can attack the substrate. To stabilize thermoplastic materials such as PVC and reduce degradation, epoxy resins may be added to the coating formulation. These epoxy resins typically include polyglycidyl ethers of aromatic polyols such as bisphenol A (often referred to as “BADGE”). Epoxy novolacs and epoxidized linseed oil have also been used as a stabilizer for coating formulations containing thermoplastic materials such as PVC.
Such conventional epoxy-containing coating formulations may include small amounts of (i) unreacted bisphenol A (“BPA”) or BADGE and (ii) low-molecular-weight components containing BPA or BADGE which, in the food packaging industry, can potentially migrate into packaged foodstuffs over time. In addition, conventional coating systems for use in packaging applications that require exposure to aggressive or corrosive food or beverage products often employ a BPA- or BADGE-containing epoxy-phenolic size coat in combination with a topcoat formulation containing thermoplastic materials such as PVC. Although the balance of scientific evidence available to date does not indicate clearly that traces of the aforementioned compounds that might be released from existing coatings pose health risks to humans, these compounds are perceived by some people as being potentially harmful to human health. Consequently, there is a desire to reduce or eliminate these compounds from food-contact coatings. However, it has been problematic to formulate thermoplastic coating formulations that exhibit very low or non-detectable levels of mobile forms of these compounds while still retaining the required coating characteristics (e.g., flexibility, adhesion, corrosion resistance, stability, etc.).
Thus, there is a continuing need for improved coating compositions.